Weight sensor

ABSTRACT

The invention provides a weight sensor ( 10 ) which includes a first electrically conductive sheet ( 12 ) electrically isolated from a second electrically conductive sheet ( 14 ) by inserts of a closed cell foamed polymeric dielectric ( 16 ) and an elastic dielectric ( 18 ) in spaces ( 20 ) formed between the inserts ( 16 ) located between the sheets ( 12, 14 ). The sensor ( 10 ) also includes capacitive measuring means ( 24 ), electrically connected between the first sheet ( 12 ) and the second sheet ( 14 ), which measures a change of capacitance between the sheets ( 12, 14 ) when a vehicle passes over the sheets ( 12, 14 ). The sensor ( 10 ) further includes converting means ( 26 ) for converting the change of capacitance to a number related to a weight of the vehicle.

FIELD OF THE INVENTION

[0001] The present invention lies in the field of weighing in motion,that is, weighing vehicles when they pass over a weight sensor. Avehicle's speed may be calculated using the apparatus in conjunctionwith additional vehicle detection means. In particular, the inventionrelates to a weight sensor and to a method of manufacturing a weightsensor.

BACKGROUND TO THE INVENTION

[0002] The dynamic effects of a vehicle passing over a weight sensor atspeed are minimised by the weight sensor protruding as little above theroad surface as possible. With weight sensors being placed on top of theroad surface this requires the weight sensors to be as thin as possible.

[0003] The Inventor is aware of weight sensors of this kind whichcomprise a sandwich construction of three electrically conductivesheets, such as aluminium sheets, separated from each other by elasticstrips, such as polyurethane (PU) or silicon strips, bonded between theinner sheet and the two outer sheets. A signal from a stable oscillatoris applied between the inner sheet on the one hand and the two outersheets on the other and a circuit measures the change of capacitancewhich results when a vehicle passes over the weight sensor. The changein capacitance results from the sheets being compressed together by theweight of a vehicle wheel or wheels on a single axle or axis which maygenerically be termed an axle set. The change in capacitance is thenconverted electronically to produce a number related to the weight ofthe axle.

[0004] Such weight sensors have stringent environmental conditions tomeet. Temperatures on road surfaces rang between sub-zero to over 50degrees centigrade, the oscillator and other circuit components mustremain reasonably stable, the PU elasticity must remain reasonablystable and the bond of the PU to the sheets must remain intact. Thsandwich must be impervious to entry of water between the sheets as thisimmediately causes leakage currents and adversely affects the securityof the bond and the stability of the oscillator. Because it is necessaryto measure weight of all axle sets of a vehicle which pass over a weightsensor within milliseconds of each other when a vehicle is at speed thePU strips must elastically recover from compression very quickly.

[0005] The Inventor is aware that these requirements have beensubstantially met by a product of this type which is moreover not undulyexpensive compared, for example, to load cell technology. However, thistype of product is manufactured by a laborious method requiring of theorder of one man-week per weight sensor. The aluminium sheets aresurface treated or primed for bonding and metal strips, which serve asmoulding cores, are prepared with a release agent. A module containingthe oscillator and circuitry and the metal strips are assembled betweenthe sheets in a jig. PU Is injected into the gaps between the metalstrips and allowed to cure over 24 to 48 hours. The metal strips arethen pulled out and the edges of the sheets are sealed by PU, appliedaround the edges, and allowed to cure.

[0006] Apart from the laborious nature of this method, quality controlin the manufacture such weight sensors is difficult and tends to beinconsistent. The weather at the time of the work causing greater orlesser humidity, impurities in the air of dust, oils or other workshoppollutants even only in parts per million which are normally not noticedaffect the quality of the bonds, as well as errors by the artisansplacing the release treated strips, etc. Bond quality is importantbecause the compression of the PU strips causes shear forces at the bondso that any weakness in the bond results in partial or complete failureof the bond in service. Once failed the response characteristics aredistorted and the weight sensor becomes unserviceable.

[0007] These problems inhibit the development of this industry frombeing able to supply substantial orders of reliable consistent qualityweight sensors to the market place.

[0008] Examination of the role played by the air gaps created betweenthe outer sheets and the inner sheet and between each pair of PU stripsreveals that despite air being conventionally accepted as providing agood dielectric for the capacitive effect which is used in the product,the primary functional importance of the air gaps is mechanical. Theyallow the PU strips space in which to expand laterally when compressedbetween the sheets and so provide the order of elastic constantappropriate in this application. Volumetrically restricted PU is toorigid. Its elastic response to compression between the sheets, which theair gaps allow, is in fact a shear response or Poisson effect.

[0009] Furthermore, the Inventor has noted that the top sheet of suchweight sensor tends to suffer damage after prolonged use. This is due toborder regions of the top sheet resting on border parts of the weightsensor. Accordingly, when a vehicle passes over the weight sensor, thecomparative rigidity of the border parts, and the compression of partsof the sheets proximate the PU strips, results in bending of the topsheet at the border region of the top sheet. Repeated bending results instress fatigue and eventual permanent damage to the weight sensor. Thisproblem inhibits the development of this industry from being able tosupply a more durable weight sensor to the market place.

SUMMARY OF THE INVENTION

[0010] According to a first aspect of the invention there is provided amethod of manufacturing a weight sensor which includes the followingsteps:

[0011] electrically isolating a first electrically conductive sheet froma second electrically conductive sheet by inserting inserts of a foamedpolymeric dielectric between the sheets;

[0012] placing an elastic dielectric into spaces formed between theinserts of the foamed polymeric dielectric to bond with the sheets; and

[0013] leaving the inserts of the foamed polymeric dielectric to remainbetween the sheets.

[0014] Th method may include repeating the prior steps of the method forsandwiching of inserts of a foamed polymeric dielectric and an elasticdielectric between the second sheet and a third electrically conductivesheet such that the second sheet is electrically isolated from the firstand third sheets.

[0015] The elastic dielectric may be chosen from those which elasticallyrecover from compression very quickly, typically in the order ofmilliseconds, and are capacitively insensitive to temperature changes ina range encountered on road surfaces. The range is typically between−20° C. and 85° C. Preferably, the elastic dielectric is also chosenfrom those which are water resistant and form a strong bond with thesheets. The elastic dielectric may be polyurethane (PU).

[0016] Placing of the elastic dielectric into spaces formed between theinserts may include injecting the elastic dielectric into the spaces.

[0017] The foamed polymeric dielectric may be a dosed cell foamedpolymeric dielectric. The closed cell foamed polymeric dielectric may beselected for good dielectric properties, for example, Neoprene(trademark) i.e. chloroprene rubber may be used. The dosed cells orpores in the inserts contain air or another gas which provides asufficient elasticity for the elastic dielectric to expand laterallydisplacing the inserts without significantly adversely affecting thefavorable elasticity properties of the elastic dielectric for thisapplication. The air or another gas in the closed cells or pores mayeven be removed to form a vacuum.

[0018] The inserts may be linear strips analogous to the air gaps in theexisting product. However, the application of such inserts in the methodaccording to this invention allows the inserts to be of any shape, notnecessarily only linear, because they do not have to be pulled out asthe metal strips of the existing product must be.

[0019] According to a preferred embodiment of the invention the insertsare of shapes that leave discrete areas, e.g. circular or roundedsquares or other shapes to be occupied by the elastic dielectric. Spruesrepassages may be left for the elastic dielectric to be injected intoafter the inserts are sandwiched between the sheets since as mentionedthe inserts remain between the sheets.

[0020] The electrically conductive sheets may be metallic sheets of goodconductivity and sufficient strength to ensure a long operational lifeof the product of the method. The sheets may be aluminium sheets. Eachsheet may include an electrically conductive film covering a plate madefrom a material which ensures a long operational life of the weightsensor.

[0021] The surfaces of the sheets in the spaces between the inserts maybe primed for good bonding with the elastic dielectric. This may be doneafter the inserts are placed on the sheets prior to or after sandwichingthem together.

[0022] The method may include electrically connecting capacitivemeasuring means between the first sheet and the second sheet so as tomeasure a change of capacitance between the sheets when a vehicle passesover the sheets. The method may include electrically connecting thecapacitive measuring means between the second sheet and the third sheet.The method may further include converting the change of capacitance to anumber related to a weight of the vehicle.

[0023] The method may include sealing the borders of the product of themethod produced thus far to make it impervious of entry of water.

[0024] The use of inserts according to the invention allows an approachwhich may ameliorate the problem of the sensitivity of the bond of theelastic dielectric to the sheets to pollution of the primed surface.

[0025] According to a second aspect of the invention there is provided amethod of manufacturing a weight sensor which includes the followingsteps:

[0026] pre-cutting a first full layer of a foamed polymeric dielectricto a desired pattern to define inserts and removable spacers;

[0027] priming a first surface of a first electrically conductive sheetfor bonding;

[0028] placing the first full layer over the first sheet immediatelyafter the first sheet has been primed thereby protecting the firstprimed surface of the first sheet;

[0029] removing the spacers;

[0030] leaving the inserts of the first full layer in place;

[0031] priming a second electrically conductive sheet for bonding;

[0032] immediately sandwiching the inserts of the first full layerbetween the first and second sheets; and

[0033] adding an elastic dielectric into the spaces formed by theremoval of the spacers so as to bond to the sheets.

[0034] The method may include repeating the prior steps of the methodfor sandwiching of inserts of a foamed polymeric dielectric and anelastic dielectric between the second sheet and a third electricallyconductive sheet such that the second sheet is electrically isolatedfrom the first and third sheets.

[0035] The foamed polymeric dielectric may be a closed cell foamedpolymeric dielectric.

[0036] According to a third aspect of the invention there is provided amethod of manufacturing a weight sensor which includes the followingsteps:

[0037] coating a first and a second side of a first full layer of afoamed polymeric dielectric with a contact adhesive or the like;

[0038] placing a first and a second protective non-stick foil againstthe contact adhesive coating the first and the second side respectively,

[0039] pre-cutting the first full layer and the foils to a desiredpattern to define inserts and removable spacers;

[0040] removing the foil from the first side of the first full layer,only in the zones which cover the inserts;

[0041] priming a first surface of a first electrically conductive sheetand a first surface of a second electrically conductive sheet forbonding;

[0042] immediately placing the first full layer having foils attachedthereto onto the first surface of the first sheet such that the firstside of the first full layer is in contact with the first surface of thefirst sheet and such that the inserts stick to the first surface, butthe spacers do not stick due to the presence there of the foil;

[0043] immediately removing the foil from a second side of the firstfull layer,

[0044] immediately removing the spacers, which include foil adhered tothe first side thereof, from the first full layer;

[0045] immediately sandwiching the inserts between the sheets; and

[0046] adding an elastic dielectric into spaces formed by the removal ofthe spacers so as to bond to the sheets.

[0047] The method may include repeating the prior steps of the methodfor sandwiching of inserts of a second full layer of a dosed cell foamedpolymeric dielectric and an elastic dielectric between the second sheetand a third electrically conductive sheet such that the second sheet iselectrically isolated from the first and third sheets.

[0048] The foamed polymeric dielectric may be a dosed cell foamedpolymeric dielectric.

[0049] The invention includes in its scope the products produced by themethods described.

[0050] According to a fourth aspect of the invention there is provided aweight sensor which includes a first electrically conductive sheetelectrically isolated from a second electrically conductive sheet byinserts of a foamed polymeric dielectric and an elastic dielectric inspaces formed between the inserts located between the sheets.

[0051] The sensor may include a third electrically conductive sheetelectrically isolated from the second sheet by inserts of a dosed cellfoamed polymeric dielectric and an elastic dielectric in spaces formedbetween the inserts located between the second and third sheets.

[0052] The elastic dielectric may be chosen from those which elasticallyrecover from compression very quickly, typically in the order ofmilliseconds, and are capacitively insensitive to temperature changes ina range encountered on road surfaces. The range is typically between−20° C. and 85° C. Preferably, the elastic dielectric is also chosenfrom those which are water resistant and form a strong bond with thesheets. The elastic dielectric may be polyurethane (PU).

[0053] The foamed polymeric dielectric may be a dosed cell foamedpolymeric dielectric. The dosed cell foamed polymeric dielectric may beselected for good dielectric properties, for example, Neoprene(trademark) i.e. chloroprene rubber may be used. The closed cells orpores in the inserts may contain air or another gas which provides asufficient elasticity for the elastic dielectric to expand laterallydisplacing the inserts without significantly adversely affecting thefavorable elasticity properties of the elastic dielectric for thisapplication. The air or another gas in the dosed cells or pores may evenbe removed to form a vacuum.

[0054] The inserts may be linear strips analogous to the air gaps in theexisting product. However, the application of such inserts in the methodaccording to this invention allows the advantage that inserts may be ofany shape, not necessarily only linear, because they do not have to bepulled out as the metal strips of the existing product must be.

[0055] According to a preferred embodiment of the invention the insertsare of shapes that leave discrete areas, e.g. circular or roundedsquares or other shapes to be occupied by the elastic dielectric. Spruesor passages may be left for the elastic dielectric to be injected intoafter the inserts are sandwiched between the sheets since as mentionedthe inserts remain between the sheets.

[0056] The electrically conductive sheets may be metallic sheets of goodconductivity and sufficient strength to ensure a long operational lifeof the weight sensor. The sheets may be aluminium sheets. Each sheet mayinclude an electrically conductive film covering a plate made from amaterial which ensures a long operational life of the weight sensor.

[0057] The inserts between each sheet may be integrally connected toeach other to form a full layer of the foamed polymeric dielectricbetween each sheet.

[0058] The sensor may include capacitive measuring means, electricallyconnected between the first sheet and the second sheet, which measures achange of capacitance between the sheets when a vehicle passes over thesheets. The capacitive measuring means may be electrically connectedbetween the second sheet and the third sheet. The sensor may furtherinclude converting means for converting the change of capacitance to anumber related to a weight of the vehicle.

[0059] Borders of the sensor may be sealed to make it impervious ofentry of water.

[0060] According to a fifth aspect of the invention, there is provided aweight sensor which includes

[0061] a top electrically conductive sheet electrically isolated from alower electrically conductive sheet by an elastic dielectric, theelastic dielectric being configured between the sheets to allow lateralexpansion of the elastic dielectric when a downward force is applied tothe weight sensor causing the sheets to compress; and

[0062] a border part of the sensor, which border part extends form aborder region of a part of the sensor located on the ground, in use, toa border region of the top sheet, wherein the border part is connectedto an upper surface of the top sheet so that the top sheet does not reston the border part.

[0063] The border part may be shaped and dimensioned to form a ramp forvehicles to drive on and off the sensor, in use.

[0064] The part of the sensor located on the ground, in use, may be thelower sheet.

[0065] The sensor may include a still lower electrically conductivesheet electrically isolated from the lower sheet by an elasticdielectric, the elastic dielectric being configured between the sheetsto allow lateral expansion of the elastic dielectric when a downwardforce is applied to the weight sensor causing the sheets to compress,wherein the part of the sensor located on the ground, in use, is aborder region of the still lower sheet. It is to be appreciated thatthis includes an embodiment of this aspect of the invention where theborder part and the still lower sheet comprise an integral unit.

[0066] The elastic dielectric may be configured between the sheets toallow lateral expansion of the elastic dielectric by way of the elasticdielectric being in the form of a plurality of linear strips separatedby air gaps. The elastic dielectric may be configured between the sheetsto allow lateral expansion of the elastic dielectric by way of theelastic dielectric being placed in spaces formed between inserts of afoamed polymeric dielectric located between the sheets.

[0067] The sensor typically includes two like border parts connected toapposing border regions of the top sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Th invention will now be d scribed, by way of a non-limitingexample, with reference to the following diagrammatic drawings, inwhich:

[0069]FIGS. 1 and 2 show in side and plan views respectively, a weightsensor in accordance with one aspect of the invention;

[0070]FIG. 3 shows in plan view, an alternate embodiment of a dosed cellfoamed polymeric dielectric in the form of a full layer for use in aweight sensor, and

[0071]FIG. 4 shows, in side view, a weight sensor in accordance withanother aspect of the invention aimed at reducing bending of the topsheet at a border region of the top sheet.

DETAILED DESCRIPTION OF THE DRAWINGS

[0072] With reference to FIGS. 1 and 2, reference numeral 10 shows aweight sensor according to the invention. The weight sensor 10 includesa first electrically conductive sheet 12 electrically isolated from asecond electrically conductive sheet 14 by inserts of a dosed cellfoamed polymeric dielectric 16 and an elastic dielectric 18 in spaces 20formed between the inserts 16 located between the sheets 12, 14.

[0073] The sensor 10 includes a third electrically conductive sheet 22electrically isolated from the second sheet 14 by inserts of a closedcell foamed polymeric dielectric 16 (shown as shaded areas in thefigures) and an elastic dielectric 18 in spaces 20 formed between theinserts 16 located between the second and third sheets 14, 22.

[0074] The first sheet 12 has a flat rectangular base 32 and raisedborder parts 34 proximate a pair of apposing borders of the first sheet12. The border parts 34 are shaped and dimensioned to receive on topthereof a pair of apposing border regions 50 of the third flatrectangular sheet 22. The second sheet 14 is a flat rectangular sheetwhich is sandwiched between the first and third sheets 12, 22. Thesheets 12, 14, 22 are aluminium sheets of good conductivity andsufficient strength to ensure a long operational life of the weightsensor.

[0075] The inserts 16 are in the form of linear strips analogous to theair gaps in the existing product. The inserts 16 are made from Neoprene(trademark). The closed cells or pores in the inserts 16 contain airwhich provides a sufficient elasticity for the elastic dielectric 18 toexpand laterally displacing the inserts 16 without significantlyadversely affecting the favorable elasticity properties of the elasticdielectric 18 for this application.

[0076] The elastic dielectric 18 is made from polyurethane (PU).

[0077] The sensor 10 includes capacitive measuring means 24,electrically connected between the first sheet 12 and the second sheet14, which measures a change of capacitance between the sheets 12, 14when a vehicle passes over the sheets 12, 14. The sensor 10 furtherincludes converting means 26 for converting the change of capacitance toa number related to a weight of the vehicle. It is to be appreciatedthat, in an alternative embodiment of the invention (not shown), thecapacitive measuring means 24 is electrically connected between thesecond sheet 14 and the third sheet 22.

[0078] Inner borders 28 and outer borders 30 of the sensor 10 are sealedto make it impervious of entry of water. The outer borders 30 aredefined by the zones of contact between the first and third sheets 12,22 and are sealed with an adhesive resin. As the inner borders 28 aresealed proximate the borders of the second sheet 14 this seal must alsobe a dielectric so as to electrically isolate the second sheet 14 fromthe first and third sheets 12, 22. PU is used to facilitate the sealingof the inner borders 28.

[0079]FIG. 3 shows an alternate embodiment of the closed cell foamedpolymeric dielectric in the form of a full layer 36 which is used toelectrically isolate the second sheet 14 from the first and third sheets12, 22. The full layer 36 is coated with a contact adhesive which iscovered by a protective non-stick foil on both sides of the full layer36. The full layer 36 and the foils are pre-cut to define its 16 andremovable spacers 38. Th spacers 38 consist of a plurality ofdisc-shaped segments 40 linked by sprues 42.

[0080] During the method of manufacturing the weight sensor 10 the foilis removed from a first side of the first full layer 36, only in thezones which cover the inserts 16. A first surface of the first sheet 12and a first surface of the second sheet 14 are then primed for bonding.The first full layer 36 having foils attached thereto is immediatelyplaced onto the first surface of the first sheet 12 such that the firstside of the first full layer 36 is in contact with the first surface ofthe first sheet 12 and such that the inserts 16 stick to the firstsurface, but the spacers 38 do not stick due to the presence there ofthe foil. The foil is then immediately removed from a second side of thefirst full layer 36 whereafter the spacers 38, which include foiladhered to the first side thereof, are removed from the first full layer36. The inserts 16 are then immediately sandwiched between the sheets12, 14 and the elastic dielectric 18 is placed, by means of injection,into spaces 20 formed by the removal of the spacers 38.

[0081] The prior steps of the method Is then repeated for sandwiching ofinserts of a second full layer (not shown) of a dosed cell foamedpolymeric dielectric and an elastic dielectric 18 between the secondsheet 14 and the a third sheet 22 such that the second sheet 14 iselectrically isolated from the first and third sheets 12, 22.

[0082] It is to be appreciated that in another embodiment of theinvention, each insert may be an open pore insert, in a suitableconfiguration. Thus sufficiently small pores may allow this to besuitable, or a material which is resistant to ingress of the elasticdielectric into the pores sufficiently. Or an open pore material whichhas a skin or dosed pore outer layer may be used.

[0083]FIG. 4 shows a weight sensor 10 in accordance with another aspectof the invention aimed at reducing bending of the top sheet 22 at borderregions 50 of the top sheet 22. It is to b appreciated that, whereappropriate in FIG. 4, like reference numerals have been used toindicate like or similar features to those shown In FIGS. 1 and 2.

[0084] The weight sensor 10 includes two border parts 52, each extendingfrom a border region 54 of the still lower sheet 12 (which is the partof the sensor 10 located on the ground, in use, in this embodiment ofthe invention) to a border region 50 of the top sheet 22. Each borderpart 52 is connected to an upper surface of the top sheet 22 so that thetop sheet 22 does not rest on the border parts 52. The border parts 52are shaped and dimensioned to form a ramp for vehicles to drive on andoff the sensor 10, in use.

[0085] It is to be appreciated that, as in FIGS. 1 and 2, the elasticdielectric 18 is configured between the sheets 12, 14, 22 to allowlateral expansion of the elastic dielectric 18 when a downward force isapplied to the weight sensor 10 causing the sheets 12, 14, 22 tocompress.

[0086] It is also to be appreciated that, in another embodiment of thisaspect of the invention, the border parts 52 and the still lower sheet12 may comprise an integral unit.

[0087] The Inventor regards it as an advantage that the inserts are notremoved after the elastic dielectric is placed within the weight sensoras the removal may cause impurities associated with the inserts toremain within the sensor. The insertion of the full layer of inserts hasthe added advantage of reducing production time as it facilitates theproper alignment of the elastic dielectric between the plates. TheInventor also regards it as an advantage that the top sheet does notrest on the border parts of the weight sensor. This should reducebending of border regions of the top sheet when a vehicle passes overthe sensor thereby reducing stress fatigue in the sensor.

1. A method of manufacturing a weight sensor which includes thefollowing steps: electrically isolating a first electrically conductivesheet from a second electrically conductive sheet by inserting insertsof a foamed polymeric dielectric between the sheets; placing an elasticdielectric into spaces formed between the inserts of the foamedpolymeric dielectric to bond with the sheets; and leaving the inserts ofthe foamed polymeric dielectric to remain between the sheets.
 2. Amethod as claimed in claim 1, which includes repeating the prior stepsof the method for sandwiching of inserts of a foamed polymericdielectric and an elastic dielectric between the second sheet and athird electrically conductive sheet such that the second sheet iselectrically isolated from the first and third sheets.
 3. A method asclaimed in either one of claims 1 or 2, wherein the elastic dielectricare chosen from those which elastically recover from compression veryquickly and are capacitively insensitive to temperature changes in arange encountered on road surfaces.
 4. A method as claimed in any one ofclaims 1 to 3, wherein the elastic dielectric is chosen from those whichare water resistant and form a strong bond with the sheets.
 5. A methodas claimed in any one of claims 1 to 4, wherein placing of the elasticdielectric into spaces formed between the inserts includes injecting theelastic dielectric into the spaces.
 6. A method as claimed in any one ofclaims 1 to 5, wherein the foamed polymeric dielectric is a dosed cellfoamed polymeric dielectric.
 7. A method as claimed in claim 6, whereinthe dosed cell foamed polymeric dielectric is selected for gooddielectric properties.
 8. A method as claimed in any one of claims 1 to7, wherein the inserts are of shapes that leave discrete areas to beoccupied by the elastic dielectric.
 9. A method as claimed in claim 8,wherein passages are left for the elastic dielectric to be injected intoafter the inserts are sandwiched between the sheets.
 10. A method asclaimed in any one of claims 1 to 9, wherein the surfaces of the sheetsin the spaces between the inserts are primed for good bonding with theelastic dielectric.
 11. A method as claimed in any one of claims 1 to10, which includes electrically connecting capacitive measuring meansbetween the first sheet and the second sheet, which capacitive measuringmeans is configured to measure a change of capacitance between thesheets when a vehicle passes over the sheets.
 12. A method as claimed inany one of claims 2 to 11, which includes electrically connecting acapacitive measuring means between the second sheet and the third sheet,which capacitive measuring means is configured to measure a change ofcapacitance between the sheets when a vehicle passes over the sheets.13. A method as claimed in either one of claims 11 or 12, which includeselectrically connecting a converting means to the capacitive measuringmeans, which converting means, in use, converts a change of capacitancemeasured by the capacitive measuring means to a number related to aweight of the vehicle.
 14. A method as claimed in any on of claims 1 to13, which includes sealing the borders of the product of the methodproduced thus far to make it impervious of entry of water.
 15. A methodof manufacturing a weight sensor which includes the following steps:pre-cutting a first full layer of a foamed polymeric dielectric to adesired pattern to define inserts and removable spacers; priming a firstsurface of a first electrically conductive sheet for bonding; placingthe first full layer over the first sheet immediately after the firstsheet has been primed thereby protecting the first primed surface of thefirst sheet; removing the spacers; leaving the inserts of the first fulllayer in place; priming a second electrically conductive sheet forbonding; immediately sandwiching the inserts of the first full layerbetween the first and second sheets; and adding an elastic dielectricinto the spaces formed by the removal of the spacers so as to bond tothe sheets.
 16. A method as claimed in claim 15, which includesrepeating the prior steps of the method for sandwiching of inserts of afoamed polymeric dielectric and an elastic dielectric between the secondsheet and a third electrically conductive sheet such that the secondsheet is electrically isolated from the first and third sheets.
 17. Amethod of manufacturing a weight sensor which includes the followingsteps: coating a first and a second side of a first full layer of afoamed polymeric dielectric with a contact adhesive or the like; placinga first and a second protective non-stick foil against the contactadhesive coating the first and the second side respectively; pre-cuttingthe first full layer and the foils to a desired pattern to defineinserts and removable spacers; removing the foil from the first side ofthe first full layer, only in the zones which cover the inserts; priminga first surface of a first electrically conductive sheet and a firstsurface of a second electrically conductive sheet for bonding;immediately placing the first full layer having foils attached theretoonto the first surface of the first sheet such that the first side ofthe first full layer is in contact with the first surface of the firstsheet and such that the inserts stick to the first surface, but thespacers do not stick due to the presence there of the foil; immediatelyremoving the foil from a second side of the first full layer;immediately removing the spacers, which include foil adhered to thefirst side thereof, from the first full layer; immediately sandwichingthe inserts between the sheets; and adding an elastic dielectric intospaces formed by the removal of the spacers so as to bond to the sheets.18. A method as claimed in claim 17, which includes repeating the priorsteps of the method for sandwiching of inserts of a second full layer ofa closed cell foamed polymeric dielectric and an elastic dielectricbetween the second sheet and a third electrically conductive sheet suchthat the second sheet is electrically isolated from the first and thirdsheets.
 19. A weight sensor which includes a first electricallyconductive sheet electrically isolated from a second electricallyconductive sheet by inserts of a foamed polymeric dielectric and anelastic dielectric in spaces formed between the inserts located betweenthe sheets.
 20. A weight sensor as claimed in claim 19, which includes athird electrically conductive sheet electrically isolated from thesecond sheet by inserts of a closed cell foamed polymeric dielectric andan elastic dielectric in spaces formed between the inserts locatedbetween the second and third sheets.
 21. A weight sensor as claimed ineither one of claims 19 or 20, wherein the elastic dielectric is chosenfrom those which elastically recover from compression very quickly andare capacitively insensitive to temperature changes in a rangeencountered on road surfaces.
 22. A weight sensor as claimed in any oneof claims 19 to 21, wherein the elastic dielectric is chosen from thosewhich are water resistant and form a strong bond with the sheets.
 23. Aweight sensor as claimed in any one of claims 19 to 22, wherein thefoamed polymeric dielectric is a dosed cell foamed polymeric dielectric.24. A weight sensor as claimed in claim 23, wherein the dosed cellfoamed polymeric dielectric is selected for good dielectric properties.25. A weight sensor as claimed in any one of claims 19 to 24, whereinthe inserts are linear strips.
 26. A weight sensor as claimed in any oneof claims 19 to 24, wherein the inserts are of shapes that leavediscrete areas to be occupied by the elastic dielectric.
 27. A weightsensor as claimed in claim 26, wherein passages are left for the elasticdielectric to be injected into after the inserts are sandwiched betweenthe sheets.
 28. A weight sensor as claimed in any one of claims 19 to27, wherein the electrically conductive sheets are metallic sheets ofgood conductivity and sufficient strength to ensure a long operationallife of the weight sensor.
 29. A weight sensor as claimed in any one ofclaims 19 to 27, wherein each sheet includes an electrically conductivefilm covering a plate mad from a material which ensures a longoperational life of the weight sensor.
 30. A weight sensor as claimed inany one of claims 19 to 29, wherein the inserts between each sheet areIntegrally connected to each other to form a full layer of the foamedpolymeric dielectric between each sheet.
 31. A weight sensor as claimedin any one of claims 19 to 30, which includes capacitive measuringmeans, electrically connected between the first sheet and the secondsheet, which capacitive measuring means is configured to measure achange of capacitance between the sheets when a vehicle passes over thesheets.
 32. A weight sensor as claimed in any one of claims 20 to 31,which includes capacitive measuring means, electrically connectedbetween the second sheet and the third sheet which capacitive measuringmeans is configured to measure a change of capacitance between thesheets when a vehicle passes over the sheets.
 33. A weight sensor asclaimed in either one of claims 31 or 32, which includes convertingmeans for converting the change of capacitance to a number related to aweight of the vehicle.
 34. A weight sensor as claimed in any one ofclaims 19 to 33, wherein borders of the sensor are sealed to make itimpervious of entry of water.
 35. A weight sensor which includes a topelectrically conductive sheet electrically isolated from a lowerelectrically conductive sheet by an elastic dielectric, the elasticdielectric being configured between the sheets to allow lateralexpansion of the elastic dielectric when a downward force is applied tothe weight sensor causing the sheets to compress; and a border part ofthe sensor, which border part extends from a border region of a part ofthe sensor located on the ground, in use, to a border region of the topsheet, wherein the border part is connected to an upper surface of thetop sheet so that the top sheet does not rest on the border part.
 36. Aweight sensor as claimed in claim 35, wherein the border part is shapedand dimensioned to form a ramp for vehicles to drive on and off thesensor, in use.
 37. A weight sensor as claimed in either one of claims35 or 36, which includes a still lower electrically conductive sheetelectrically isolated from the lower sheet by an elastic dielectric, theelastic dielectric being configured between the sheets to allow lateralexpansion of the elastic dielectric when a downward force is applied tothe weight sensor causing the sheets to compress, wherein the part ofthe sensor located on the ground, in use, is a border region of thestill lower sheet.
 38. A weight sensor as claimed in any one of claims35 to 37, wherein the elastic dielectric is configured between thesheets to allow lateral expansion of the elastic dielectric by way ofthe elastic dielectric being in the form of a plurality of linear stripsseparated by air gaps.
 39. A weight sensor as claimed in any one ofclaims 35 to 38, wherein the elastic dielectric is configured betweenthe sheets to allow lateral expansion of the elastic dielectric by wayof the elastic dielectric being placed in spaces formed between insertsof a foamed polymeric dielectric located between the sheets.
 40. Aweight sensor as claimed in any on of claims 35 to 39, which includestwo like border parts connected to apposing border regions of the topsheet.
 41. A product produced by a method as claimed in any one ofclaims 1 to
 18. 42. A method, a weight sensor, or a product according tothe invention, as hereinbefore generally described.
 43. A method, aweight sensor, or a product as specifically described with reference toor as illustrated in the accompanying drawings.